OK; you might find this Puget Systems blog post useful if you haven't already seen something like it. I've never built a machine using the higher end CPUs, but differentiating between models always seemed a bit messy to me. That post helped make things clearer.
I'd strongly recommend going with a build from the buyers' guide. Depending on what you want to use your machine for, those parts might be much more than you need (particularly the CPU). If you post your intended usage here, people will probably be able to make some suggestions.
I don't do video editing, so can't really comment on the appropriateness of your CPU and motherboard selection, but there are plenty of existing video editing builds listed on this website. What I can suggest is considering your budget and SSD choice(s). I'd expect that a larger but marginally...
1. Try checking the recommended RAM in the Buyers' Guide.
2. Separate OSs on separate drives is viable, but have you checked on the compatibility of your motherboard with MacOS? (If not, try searching for the same motherboard on this website.)
Hope this helps.
You could save money by just using the built-in GPU on your CPU (for now) and only buy a discrete GPU card later. Unless you have a specific reason for needing an M.2 device (it sounds as if you don't), then you could use a motherboard without it. Without a discrete GPU card, you could probably...
As a suggestion, see the CustoMac Nano section in the buyers' guide linked above for a 'bare minimum' build, and scale up from there until you reach your budget (e.g., CustoMac Mini would be the next step up).
Check the buyers' guide (linked above) for parts compatibility. For PSU capacity, you can check various on-line power usage calculators, but looking at your list, I'd guess that anything from 550 W up should be fine.
You've listed a CPU cooler (Scythe Mugen 4) under your PSU heading, but assuming you're using any decent PSU (e.g., Corsair, Seasonic) and you've checked all the physical clearances (e.g., GPU length), your list looks good to me.
(1) You should be able to fit 64 GB on that motherboard.
(2) In terms of outright cooling, I believe that liquid-cooling systems give better performance, but are also noisier than air-cooling systems.
(3) There are various power usage calculators available, but 650 W looks fine to me for the...